The present PEN homopolymer and copolymer invention generally relates to a dilation catheter with an inflatable balloon which expands against the internal walls of a vessel to open or dilate a constriction, stenosis or occlusion of the vessel and, percutaneous transluminal angioplasty (PTA).
The PTA procedure places unique demands on the types of materials needed to fabricate a catheter fitted with an expandable balloon. The physical properties and characteristics of a desirable balloon may result in certain characteristics being balanced against others. For instance, very strong thermoplastic materials that are sufficiently strong enough to allow for minimum balloon wall thicknesses tend to be rigid, hard or stiff compared to more elastomeric materials that tend to be flexible, soft and deformable. Using stronger materials may give a minimum profile balloon but the stiffness of the material may be more likely to injure or traumatize the vascular system as the balloon is positioned to and then across a stenosis or occlusion.
In the art of catheter balloon manufacture, a variety of thermoplastic polymers have been used to prepare balloon structures. Polyethylene terephthalate (PET) of varying intrinsic viscosities have been widely commercialized.
In the past, PTA catheter balloons have been made from polymeric materials which gave balloons that may be broadly categorized into two groups: a) non-compliant balloons and b) compliant balloons.
Non-compliant balloons typically unfold to a nominal diameter and then stretch or expand only slightly beyond that diameter as the pressure is increased. See Levy, U.S. Pat. No. Re 32,983, Wang U.S. Pat. No. 5,195,969 and Wang U.S. Pat. No. 5,330,428. All three patents describe biaxially oriented polyethylene terephthalate (PET) balloons. In comparison compliant balloons typically inflate to a nominal diameter and then continue to stretch or expand as the inflation pressure is increased until the strength of the balloon material is exceeded and the balloon bursts. See Becker U.S. Pat. No. 4,154,244 and Wang, et al, U.S. Pat. No. 5,556,383.
In U.S. Pat. No. 5,270,086 it is proposed that a multilayer balloon could be made with an outer layer of a high tensile strength polymer and an inner bonding layer of a highly distensible polymer which had good melt bond and glue adhesion properties. Among the various materials proposed for the outer layer is polyethylene naphthalate. This reference, however, only exemplifies balloons in which the tensile layer is PET.
It has also been suggested that catheter balloons could be made of polyester/polyether block copolymers in which the polyester blocks were polyesters of naphthalene dicarboxylic acid (U.S. Pat. No. 5,556,383). To date, however, it has not been suggested that balloons made from polyethylene naphthalate could be formed in a manner to give properties significantly different from those of prior art balloons and it has not been demonstrated that such balloons could have substantially improved properties relative to commercial materials such as PET.
It has now been discovered that medical device balloons whose average strength is substantially higher than commercially standard PET balloons can readily be prepared from certain polyethylene naphthalate (PEN) homopolymers or copolymers. The high average strength of the PEN balloons is obtained even without exclusion of balloons having cosmetic defects, such as fish eyes or mottled surfaces.
The inventive catheter balloons have a structural layer of a polymer material of PEN homopolymer, or of a PEN copolymer which is a crystallizable copolyester made up of residues of ethylene glycol, of naphthalene dicarboxylic acid (NDC), and of a second dicarboxylic acid (PA) which is terephthalic acid or isophthalic acid, or a mixture thereof. The balloons are characterized by an ability to withstand a hoop stress of at least 35,000 psi without bursting, and can readily be designed to have a hoop stress of at least 50,000, even for larger diameter balloons having nominal diameters in the range of 6-30 mm.
The balloons of the invention may be either single layer balloons, or multilayer balloons. In one preferred embodiment the balloon comprises an inner layer of PEN homopolymer or copolymer and an outer layer of a polybutylene naphthalate polymer or copolymer.